Field
The present disclosure relates generally to energy storage devices, and more particularly to electrolytes for metal-ion battery technology and the like.
Background
Owing in part to their relatively high energy densities, light weight, and potential for long lifetimes, advanced metal-ion batteries such as lithium-ion (Li-ion) batteries are desirable for a wide range of consumer electronics. In many applications, Li-ion batteries have essentially replaced nickel-cadmium and nickel-metal-hydride batteries. Despite their increasing commercial prevalence, however, further development of metal-ion batteries is needed, particularly for potential applications in low- or zero-emission hybrid-electrical or fully-electrical vehicles, consumer electronics, energy-efficient cargo ships and locomotives, aerospace, and power grids. Such high-power applications will require electrodes with higher specific capacities than those used in currently-existing Li-ion batteries.
Currently, carbon-based materials (e.g., graphite) are employed as the predominant anode material in Li-ion batteries. Carbon (C), in the form of graphite, has a maximum or theoretical specific capacity of about 372 milli-Ampere hours per gram (mAh/g). A variety of higher capacity materials have been investigated to overcome the drawbacks of carbon-based materials. Materials that electrochemically alloy with Li and their composites have received great attention as anode candidates because they exhibit specific capacities that are several times greater than that of conventional graphite. Silicon, for example, has the highest theoretical specific capacity among alloying materials, topping out at about 4200 mAh/g.
Conventional implementations of such alloying-type anodes and their composites, however, have been hindered by several problems, including large irreversible capacity losses and relatively low Coulombic Efficiency (CE) during cycling due to relatively large volume changes during cycling and the resulting poor stability of the so-called solid electrolyte interphase (SEI). The SEI comprises products of the electrolyte decomposition on the electrode during cell operation and includes a significant content of Li. In an ideal case, a Li-ion permeable (while electron and solvent impermeable) SEI forms once during the first cycle and remains stable during subsequent cycling. The relatively fast growth of the SEI in high capacity anodes (such as those that comprise alloying-type active material, such as Si) leads to the degradation of cell performance due to irreversible capacity losses (due to more and more Li being trapped within the growing SEI).
The SEI growth is primarily caused by permeation of the electrolyte solvent through the existing SEI to the active material surface at low potentials, followed by its decomposition and the addition of the new decomposed layer to the existing SEI. Alloying-type anodes and their composites often exhibit changes in the outer surface area during metal-ion insertion and extraction, inducing defects in the SEI, through which the undesirable diffusion of the electrolyte solvent may take place.
Some high capacity cathodes also have undesirable reactions with electrolytes. For example, conversion-type sulfur (S)-containing or selenium (Se)-containing cathodes may exhibit dissolution in the electrolyte of their intermediate reaction products (such as lithium polysulfides in the case of Li-ion batteries with sulfur-based cathodes or sodium polysulfides in the case of Na-ion batteries with sulfur-based cathodes). Other high capacity cathodes, such as metal fluorides (iron fluorides, copper fluorides, cobalt fluorides, bismuth fluoride and other transition metal fluorides, their alloys and their mixtures) and metal fluoride-based components, similarly suffer from volume changes and undesirable reactions of electrolytes with the components of the cathodes (such as reactions of electrolyte and metal components of the cathodes).
Some high voltage intercalation-type cathodes (which operate in a potential range up to around 4.6 V vs. Li/Li+) and some very high voltage intercalation-type cathodes (which operate in a potential range up to around 5.5 V vs. Li/Li+) are known to induce oxidation of electrolytes that are compatible with the majority of anode materials (such as carbon, silicon, tin, aluminum, and others). This electrolyte oxidation leads to gassing and rapid cell degradation. As a result, most conventional Li-ion battery cells operate up to around 4.2-4.3 V. Higher voltage cells are desirable, but presently unstable and unsafe.
Accordingly, despite the advancements made in electrode materials, high capacity metal-ion batteries remain somewhat limited in their application and there remains a need for improved batteries, components, and other related materials and manufacturing processes.